The human ear is very sensitive to noise and particularly sensitive to distortion in audio signals. For example, to maintain auditory purity, a high fidelity (HIFI) audio signal must be transmitted, encoded, decoded, and reproduced with a very high dynamic range (100 to 120 dB) and carry very little distortion (−90 to −110 dB). This level of fidelity is particularly challenging in situations where the audio signal must be transmitted between two audio integrated circuits (ICs) resident on a compact and densely populated printed circuit board (PCB). Under such conditions, a transmitted HIFI audio signal is subject to substantial noise and interference. Further, tight PCB layouts make it very difficult to isolation the HIFI signal from interference using spacing, routing, and grounding techniques.
An effective method of transmission of HIFI audio signals on a densely-populated PCB should have little or no loss of signal quality, should not be sensitive to an interfering medium, and should minimize the introduction of interference. In addition, if a transmission protocol is used, then the protocol should require minimal circuit overhead on the interconnected IC devices and should be flexible. From a PCB routing standpoint, the smallest HIFI audio signal overhead is achieved by transmitting the signal over a single wire. However, single wire transmission is very susceptible to noise pickup and must be very well isolated from other PCB conductors. The necessary isolation distances and grounding make single wire transmission of analog HIFI audio signals unsuitable for compact and densely populated PCBs. As an alternative, analog HIFI audio may be transmitted across the PCB via differential signals. This approach improves noise rejection. However, differential signals require at least two wires and increase the area used.
As an alternative, HIFI audio signals may be transmitted in a pulse width modulated (PWM) form. A variable width pulse is transmitted where the width of each pulse encodes a sampled value of the amplitude of the audio signal for that pulse period. A PWM signal may be transmitted on a single wire, is easily decoded at the receiver with a low-pass filter, and is more robust to amplitude interference. However, a PWM signal is sensitive to mismatch in the rise and fall time constraints of its pulse. A differential form of the PWM signal would resolve the mismatch issue but requires more than one wire.
A most robust transmission method is to encode a HIFI audio signal into a digital signal, transmit this signal across the PCB channel, and then decode the signal back into an analog signal. Digitally encoded amplitude information may be perfectly transmitted, received, and decoded. However, a clock is necessary to reproduce the HIFI audio signal from the digital signal at the receiving end. In addition, if there is any jitter on this clock, then this jitter will adversely affect the quality of the sound reproduced. To prevent clock jitter, the transmitted signal must be isolated from environmental amplitude noise or interference that is converted into edge timing errors through the slope of the clock pulse.
Typically, HIFI audio signals are encoded as either 20-bit, 24-bit, or 32 bit words transmitted at rates of either 48 KHz or 44.1 KHz. An exemplary protocol, like I2S, is capable to transmits digitally encoded, HIFI audio as serialized digital audio words. However, I2S is not particularly useful for densely packed PCB application because the physical layer requires separate data, word clock, and bit clock lines. Further, the I2S scheme requires a large overhead for both transmitting and receiving ICs.
Another proposed solution for transmitting HIFI audio signals via digital encoding is the Sony/Philips Digital Interconnect Format (SPDIF). In the SPDIF scheme, an analog signal is encoded as a series of digital words. These digital words are transmitted in a single-bit stream with unique headers marking the start of each word. A decoding clock is mixed into the data stream using bi-phase mark code (BMC). This clock must later be recovered from the BMC stream, typically by using a phase-locked loop (PLL), so that the clock may be used to reproduce the audio signal via a digital-to-analog converter (DAC) at the receiving end. The SPDIF physical layer may be implemented as a single wire. However, this protocol requires significant overhead for transmitting and receiving circuits, plus a PLL, a serialization circuit, a de-serialization circuit, and a decoder.
In addition to the large overhead requirements, another major issue reported with the SPDIF format is that the clock introduced in the BMC encoding is effectively modulated by the transmitted audio data. Referring now to FIG. 1, a timing diagram 10 of the operation of the prior art SPDIF audio data transmission scheme is shown. In the SPDIF protocol, the CLOCK 14 and DATA 18 are combined to form the BMC ENCODED signal 22. The BMC ENCODED signal 22 includes positive transitions that are are triggered at the rising edge of the CLOCK 14 if the DATA 18 is a series of consecutive logical “1's.” However, the BMC ENCODED signal 22 is triggered at alternating edges if the DATA is a series of consecutive logical “0's.”
It has been found that the rising edges 26 and falling edges 30 of the BMC ENCODED signal 22 are inevitably mismatched. This mismatch effects the clock signal that is recovered from the BMC ENCODED signal when it is decoded by the receiving IC. In particular, clock jitter (more precisely, aperture jitter) is introduced. Further, this clock jitter is dependent on the encoded audio signal. As a result, when the received BMC ENCODED signal 22 is decoded and reproduced by a digital-to-analog converter (DAC), the HIFI audio is audibly effected. The jitter effect can be heard as poor quality audio. The same problem arises if only the positive edges (or only the negative edges) of the BMC ENCODED signal 22 are detected. The same problem arises if Manchester encoding is used. In addition, a single line transmitting HIFI audio using the SPDIF protocol and physical layer will emit electromagnetic interference (EMI). Tones are found to be emitted from the signal line at frequencies other than the audio content. Further, it is found that adjacent SPDIF lines (as might be the case in a stereo application) are susceptible to crosstalk between the signals and to signal distortion. Although actual digital data errors are highly unlikely, this crosstalk effect is found to be audible in the recovered audio signal due to coupling of signal content onto the recovered clock. For this reason, signal lines carrying SPDIF-encoded content must typically be shielded for HIFI applications.
Another method of transmitting audio data is pulse density modulation (PDM). For example, a single-bit, analog-to-digital converter (ADC) may be used to convert an analog signal into an over-sampled single bit stream. With sufficient over-sampling and quantization noise shaping, an audio signal may be transmitted with high fidelity over a single wire. The PDM signal may then be decoded using a digital-to-analog converter (DAC) at the receiving end. Since the analog data is encoded in the PDM signal as a single-bit stream, no digital word synchronization or serialization is required. In asynchronous PDM (APDM), the encoding clock is recovered from the transmitted PDM data using a PLL, as is the case with SPDIF. Unfortunately, APDM encoding suffers the same problems of signal modulation onto the recovered clock as found in the BMC encoding or Manchester encoding. A stereo PDM (SPDM) scheme may be used to resolve the issue by supplying the clock separately. However, this scheme will not work with a single wire.
It is therefore very useful to provide a method and device for transmitting HIFI audio signals over a single wire. In the present invention, a novel and robust method to transmit HIFI content over a single wire is described. The invention allows high-fidelity audio signals to be transmitted and received on a printed circuit board via a single wire. The present invention eliminates data modulation of the system clock and minimizes signal-dependent EMI. In addition, the spectrum of signal-related transitions does not include frequency multiples of the audio signal. Further, the present invention eliminates the need for header synchronization. Finally, the invention improves PCB signal routing flexibility while removing the need for a PLL circuit.